I've heard that this is a common Contour problem... Mine did that the first week I had it, and I was concerned...but it went out and now it just stays on for a bit longer than it should when I start the car but when I start driving it goes out...and it will come on every so often when I brake hard or something... screwy Fords...
And when I was throwing money at my steering issues, I told the dealership (not a Ford stealership, a Subaru dealership that sold me this used car) to check the ABS codes as well, and they said they didn't find anything. I thought the sensor had come loose or something at first as well, because I have heard of that happening on other cars..but yeah, no major issues with that one... I wouldn't worry.
Wish I could have my car back and it won't break again NEXT week (refer to post: started, sputtered, died, won't start)...
Edit: BTW, if there is a definite wrong feeling in the brakes, like they don't work well or something, have THAT looked at, but just a light coming on, that's what I wouldn't worry about, unless the ABS doesn't seem to be working. Do what these other guys said and get the code checked and then reset the computer.